Mechanism and Capacity of Zerovalent Irons to Remediate Cr(VI) Contaminated Water Jae E. Yang*, Jong-Sung Kim1, Yong-Sik Ok and Dong Kook Kim Department of Biological Environment, Kangwon National University, Chuncheon 200-701, Korea; 1Interdisciplinary Graduate Programs, University of Iowa, Iowa City, Iowa, USA  XRD Analysis ABSTRACT To assess the mechanism of ZVI on Cr[VI] reduction, instrumental analyses such as SEM/EDS, XRD and XPS were conducted. The efficiency was assessed by the toxicity bioassay using the luminescent bacteria (Microtox®). SEM/EDS analyses indicated that ZVI with higher reducing capacity was more subject to the changes of surface and morphological properties, due to the ionization of the ZVI by reacting with oxygen and water. The precipitates were composed of the oxidized Fe and the reduced Cr mixtures. XRD analysis showed that Cr[VI] was reduced to Cr[III], when the reactive ZVI (Fe0) was oxidized to Fe[II, III], showing the presence of Fe2O3, (Fe-Cr)2O3, and FeOOH. With the increased rate of Cr[VI] reduction, oxidation of the ZVI was further increased. The XPS analysis indicated that both Cr and Fe in the precipitates were exclusively in the trivalent [III] oxidation state with the respective forms of Cr(OH)3 or Cr2O3, and FeOOH or Fe2O3. The toxicity of the reduced Cr by ZVI to Photobacterium Phosphoreum was significantly lower than that of the Cr[VI]. With the complete reduction of Cr[VI], no toxicity was observed. MATERIALS & METHODS Exp. I : Mechanistic Evidence of Cr[VI] Reduction by ZVI  ZVIs screened as reductants for Cr[VI] :  J (Shinyo Pure Chemicals, Japan), PU (Peerless Unannealed, Detroit, MI, USA), PA (Peerless Annealed, Detroit, MI, USA), F (Fisher Scientific, PA, USA), and S ZVI (Hoganas Iron, Sweden)  Among these ZVI, J and PU were the most effective in Cr[VI] reduction.  In this poster, effect of J and PU ZVI on Cr[VI] reduction were mostly included.  To assess the mechanism of ZVI on Cr[VI] reduction, instrumental analyses were conducted.  Instrumental analyses : Surface properties and chemical composition of the Fe and Cr products  Experiment Conditions : ZVI 5% (w/v) + Cr[VI] 1 mM, Reaction time 48 hr  SEM/EDS (Scanning Electron Microscope/ Energy Dispersive X-ray Spectrometer) Analysis  XRD (Powder X-ray Diffraction) Analysis  XPS (X-ray Photoelectron Spectroscopy) Analysis. Fig.3. Powder X-Ray Diffraction Pattern of ZVI Treatment.  XRD analysis showed that Cr[VI] was reduced to Cr[III], when the reactive Fe0 was oxidized to Fe[II, III]. The powder XRD patterns revealed the presence of Fe2O3, (FeCr)2O3, and FeOOH. With the increased rate of Cr[VI] reduction, oxidation of the ZVI was further increased.  XPS Analysis Exp. II : Effectiveness of the ZVI for Cr[VI] Reduction: Bioassay  Zero-Valent Iron (ZVI) : J, PU, PA, F, and S  Effectiveness of the ZVI for Cr[VI] reduction was bioassayed by Microtox® analyzer using the luminescent bacteria (Photobacterium Phosphoreum) Fig. 4. XPS Spectra of the Oxidized ZVI.  XPS analysis indicated that the Cr in the precipitate was exclusively in the Cr[III] oxidation state in the forms of Cr(OH)3 or Cr2O3, and that the Fe present in the precipitate was in the Fe[III] oxidation state with the forms of FeOOH or Fe2O3. RESULTS & DISCUSSIONS Exp. I : Mechanistic Evidence of Cr[VI] Reduction by ZVI Exp. II : Effectiveness of the ZVI for Cr[VI] Reduction: Bioassay 1. Instrument Analyses  SEM/EDS Analysis Efficiency of the ZVI Treatment for Cr[VI] Reduction and Toxicity Treatment Reduction Efficiency % Toxicity (EC50) mg L-1 Cr[VI] only 16.89 PA 74 103.7 ZVI only - Non-toxic F 65 68.23 J 100 S 29 35.75 PU J control J after 48 hr PU control PU after 48 hr  Toxicity was decreased with increasing the rate of Cr[VI] reduction.  With the complete reduction of Cr[VI], toxicity was not observed. Fig. 1. Scanning Electron Micrograph of ZVI before and after Reactions (20KV X 350). CONCLUSIONS Reduction efficiency of Cr[VI] was varied with kinds of ZVI. ZVIs of J and PU reduced 100 % and 98% of Cr[VI], respectively, within 3 hours of reaction. pH of the reacting solution of ZVI and Cr was rapidly increased to 4.34 ∼ 9.04 within 3 hours. Redox potential (Eh) was dropped from 600 mV to 319 mV within 3 hours. Decrement in Eh was greater for ZVI with a higher Cr[VI] reduction capability. Results from batch and instrumental analyses indicated that the electron produced from ZVI oxidation reduced Cr[VI] to Cr[III], thus resultantly Cr[III] precipitated or co-precipitated with Fe [III] to form Fe[III]-Cr[III] hydroxide or Fe[III]-Cr[III] oxyhydroxide. Toxicity bioassay using the luminescent bacteria proved that ZVI treatment on Cr[VI] reduced the toxicity of Cr effectively showing that toxicity was decreased with increasing the rate of Cr[VI] reduction. With the complete reduction of Cr[VI], toxicity was not observed and Iron metal can reductively precipitate anions and oxyanions, such as converting soluble Cr[VI] oxides to insoluble Cr[III] hydroxides. The reduction of Cr[VI] by ZVI produces ferric iron Fe[III] and Cr[III]. Chromium may be removed through the precipitation or co-precipitation of mixed Fe[III]-Cr[III] hydroxide solid solution. Fig. 2. Scanning Electron Micrograph with EDS Spectrum of ZVI Treatment.  SEM/EDS analyses indicated that ZVI with higher reducing capacity was more easily subject to the changes of surface and morphological properties, due to the ionization of the ZVI by reacting with oxygen and water. The precipitates were composed of the oxidized Fe and the reduced Cr.